1. Field of the Invention
This invention relates to the reduction of noxious automotive emissions, and more particularly to the addition of air to the exhaust gas stream of an automotive engine to combust unburned and partially burned hydrocarbons.
In order to meet Governmental emissions standards for internal combustion engine exhaust, motor vehicle manufacturers emplace catalytic converters in the exhaust gas lines of their vehicles. A common form of converter comprises a catalyst member which comprises a honeycomb monolith having gas flow passages extending therethrough. The monolith carries a coating of catalytically active material which is effective to convert noxious components of the exhaust gas, which may include unburned hydrocarbons, carbon monoxide and NO.sub.X to innocuous substances. A common type of catalytic material comprises catalytically effective amounts of platinum group metals dispersed on a refractory inorganic oxide support material such as alumina, ceria and zirconia. Three-way catalysts are known for their ability to substantially simultaneously oxidize unburned hydrocarbons and carbon monoxide to CO.sub.2 and H.sub.2 O while reducing NO.sub.X.
Three-way catalysts, like more conventional oxidation catalysts, are generally not effective until they have been heated to a threshold temperature often identified as the "light-off" temperature. Ordinarily, during the operation of an automotive engine, the exhaust gases heat the catalytic converter to the light-off temperature within a few minutes of operation. However, during those initial minutes of operation known as the "cold-start" period, the engine and the exhaust system apparatus are cold, so the exhaust gases are relatively cold and quickly transfer the little heat they contain to heat the catalytic converter and other components of the exhaust system. During the cold-start period, the exhaust gases are rich in unburned hydrocarbons, which pass through the cold catalytic converter unaffected. Recently, efforts have been made to reduce cold-start emissions, including incorporating an adsorbent hydrocarbon trap in the exhaust gas line. Such traps allow the exhaust gases to flow in contact with an adsorbent material, e.g., a molecular sieve, which adsorbs and thus retains the hydrocarbons during the cold-start period. When the adsorbent is heated, however, it releases the previously trapped hydrocarbons. By that time, however, the engine will be heated to its steady operation temperature and the catalytic converter will be at or near its light-off temperature, so that at least some of the desorbed hydrocarbons, which otherwise would have passed through the catalytic converter untreated, will be oxidized to less noxious species. However, since engines are generally run on a stoichiometric air/fuel combustion mixture, the desorption of hydrocarbons into the exhaust gases causes the exhaust gases to become fuel-rich, and thus prevents optimum catalyst performance.
2. Related Art
The prior art teaches that three-way catalysts are best used for treating CO, unburned hydrocarbons and NO.sub.X by maintaining the air/fuel ratio of the engine combustion mixture at a stoichiometric balance, i.e., a balance in which there is sufficient oxygen to fully combust the hydrocarbons without leaving unreacted oxygen. This allows for the complete combustion of the carbonaceous components of the fuel to proceed substantially simultaneously with the reduction of NO.sub.X. Accordingly, automobiles are equipped with engine control modules that control fuel injectors to attain the desired air/fuel balance. However, variations from stoichiometric air/fuel operation frequently occur, e.g., during acceleration and deceleration. It is generally accepted that when the combustion mixture is lean, there will be excess oxygen in the exhaust gas, and the activity of the three-way catalyst will favor oxidation of hydrocarbons and carbon monoxide. See, e.g., U.S. Pat. No. 4,171,287 to Keith at column 11, lines 29-47, where the conventional air/fuel ratio index variable A is used to relate a given air/fuel mixture to a stoichiometric air/fuel mixture, which has an air/fuel weight ratio of 14.65 for a fuel with H/C ratio of 1.90. For a lean air/fuel mixture, .lambda.&gt;1; for a stoichiometric mixture, .lambda.=1; for a fuel-rich mixture, .lambda.&lt;1.
In SAE Paper 930739, Hochmuth et al disclose in FIG. 8 a number of exhaust configurations in which a hydrocarbon trap is disposed between catalyst zones defined by discrete catalytic converters or passes of a heat exchange cross-flow monolith having three-way catalyst material in both passes. The Paper teaches the addition of air into the exhaust gas stream to burn desorbed hydrocarbons, and teaches the injection of air at a point downstream from the hydrocarbon trap to assist in the combustion of desorbed hydrocarbons in a catalyst zone further downstream from the trap.
U.S. Pat. No. 3,929,418 to Wood, dated Dec, 30, 1975, discloses a crossflow catalyst monolith mounted in a canister, in which the monolith defines a first catalyst zone for the reduction of nitrogen oxides and a second catalyst zone for the oxidation of carbon monoxide and unburned hydrocarbons, and teaches the injection of air into the exhaust gas stream at a point between the catalyst zones, to assist in oxidation in the second catalyst zone. Similar arrangements are taught in U.S. Pat. No. 3,860,535 (see column 3, lines 24 through 42) and U.S. Pat. No. 3,929,419 (see column 3, lines 51 through 63).
U.S. Pat. No. 5,051,244 to Dunne et al, dated Sep. 24, 1991, shows a valve-operated exhaust system in which, during the cold-start period of engine operation, exhaust gases are flowed through an adsorbent and then through a three-way catalyst. When the catalyst reaches its light-off temperature, the valves are used to bypass the adsorbent zone. A minor part of the hot exhaust gases is used to desorb hydrocarbons from the adsorbent and to flow them to the catalyst. There is no suggestion regarding the addition of air into the exhaust gas stream other than that which is required to combust the fuel.
Some automobiles employ a "fuel cut" mode of engine operation during deceleration, in which the fuel injectors are completely closed, forcing a large excess of air through the exhaust system which can combust adsorbed hydrocarbons.